Police will seek the cancellation of a Kapiti Coast dairy's liquor licence after it sold alcohol to underage teens twice in the same night.

The Raumati Beach Four Square was one of four outlets on the coast busted during a controlled-purchase operation using two volunteers aged 15 and 16 on Friday night, Sergeant Jacquie Muir said.

Of the 15 outlets visited, the dairy and three outlets with on licences - two cafe styled restaurants and a tavern - sold liquor to the volunteers, Ms Muir said.

In two previous operations earlier this year underage volunteers were denied alcohol at all outlets visited so the latest result was disappointing, she said.

A Four Square in Raumati Beach had previously been caught selling alcohol to minors, which was the reason police would ask the Liquor Licensing Authority to cancel its licence.

Police would apply to the authority for the other outlets' licences to be suspended.

The people who sold alcohol to the underage volunteers could be charged with selling alcohol to minors, she said. Retro Bar - under scrutiny in the wake of the deaths of two teens in Paraparaumu - was visited, but did not break the law.

Senior Sergeant Mark Buttar said two Wellington bars on Courtenay Pl and Allen St were also caught selling alcohol to underage teens at the weekend.

Both bars would also be referred to to the authority for prosecution, Mr Buttar said.

"It's slackness on their [the bars] behalf. They just need to be aware that we are out there and we are taking a hard line with alcohol harm related offending."